Growth of high-quality YBa2Cu3O7−<i>x</i> films on CeO2 buffer of mixed (001)/(111) orientation on sapphire

Abstract

High-quality epitaxial YBa2Cu3O7−x (YBCO) thin films are grown on large area (1ub;-21ubx02) sapphire (r-cut) substrates buffered by CeO2 of mixed (001) and (111) orientation. It is shown that (001) and (111) CeO2 epitaxy on (1ub;-21ubx02) sapphire is possible with further growth of well-oriented (001) YBCO on both orientations of CeO2. In the case of (111) CeO2 one of the equivalent in-plane trigonal directions, i.e., [10ub;-21ubx], [ub;-21ubx10], or [01ub;-21ubx], is aligned along either [02ub;-22ubx1], or [2ub;-22ubx01] crystallographic axis of sapphire, which is indicated by the φ-scan x-ray diffraction pattern of the (115) CeO2 reflex. The resulting (001) YBCO film grown on the (001)/(111) CeO2 buffer reveals no disorientations in the a-b plane other than conventional 90° twinning. The YBCO films exhibited very smooth surface and were free of screw dislocations. The high quality of the YBCO films is confirmed by the microwave surface resistance of Rs≤0.44 mΩ at 10 GHz and 77 K.